The present invention generally relates to a method of producing a support for a planographic printing plate, and particularly relates to a method of producing an aluminum support which is superior in an electrolytically graining property.
Conventionally, an aluminum plate (including aluminum alloy) has been used as a printing plate, such as an offset printing plate. Usually, in offset printing, it is necessary to apply a suitable adhesion and a suitable amount of water between the surface of the aluminum plate and a photosensitive layer.
The surface of the aluminum plate should be uniformly and finely grained to meet the aforesaid requirements. This graining process largely affects a printing performance and a durability of the printing plate upon the printing process following manufacture of the plate. Thus, it is important for the manufacture of the plate whether such graining is satisfactory or not.
In general, an alternating current electrolytic graining method is used as the method of graining an aluminum support for a printing plate. There are a variety of suitable alternating currents, for example a sinewaveform, a squarewaveform, a special alternating waveform and the like. When the aluminum support is grained by alternating current supplied between the aluminum plate and an opposite electrode such as a graphite electrode, this graining is usually conducted only one time, as the result of which, the depth of pits formed by the graining is small over the whole surface thereof. Also, the durability of the grained printing plate during printing will deteriorate. Therefore, in order to obtain a uniformly and closely grained aluminum plate satisfying the requirement of a printing plate with deep pits as compared with their diameters, a variety of methods have been proposed as follows.
One method is a graining method to use a current of particular waveform for an electrolytic source (Japanese Patent Laid-Open No. Sho 53-67507). Another method is to control a ratio between an electricity quantity of a positive period and that of a negative period at the time of alternating electrolytic graining (Japanese Patent Laid-Open No. Sho 54-65607). Still another method is to control the waveform supplied from electrolytic source (Japanese Patent Laid-Open No. Sho 55-25381). Finally, another method is directed to a combination of current density (Japanese Patent Laid-Open No. Sho 56-29699).
Further, known is a graining method using a combination of an AC electrolytic etching method with a mechanical graining method (Japanese Patent laid-Open No. Sho-55-142695).
As the method of producing an aluminum support, on the other hand, known in a method in which an aluminum ingot is melted and held, and then cast into a slab (having a thickness in a range from 400 to 600 mm, a width in a range from 1000 to 2000 mm, and a length in a range from 2000 to 6000 mm). Then, the thus cast slab is subject to a surface-cutting step in which the slab surface is cut off by 3-10 mm with a surface cutting machine so as to remove an impurity structure portion on the surface. Next, the slab is subject to a soaking treatment step in which the slab is kept in a holding furnace at a temperature in a range from 480.degree. to 540.degree. C. for a time in a range from 6 to 12 hours, thereby to remove any stress inside the slab and make the structure of the slab uniform. Then, the thus treated slab is hot-rolled at a temperature in a range from 480.degree. to 540.degree. C. to a thickness in a range from 5 to 40 mm. Thereafter, the slab is cold-rolled at the room temperature to a predetermined thickness. Then, in order to make the structure uniform and improve the flatness of the plate, the thus treated slab is annealed thereby to make the rolled structure, etc. uniform, and the slab is then subject to correction by cold-rolling to a predetermined thickness. Such an aluminum plate obtained in the manner as described above has been used as a support for a planographic printing plate.
Currently, the quantity of production of planographic printing plates increases with growth of demand therefor, and a large quantity of planographic printing plates having a constant quality are required to be produced. The electrolytic graining treatment is, however, apt to be affected particularly by an aluminum support to be grained. In the case of producing an aluminum support through the foregoing process, including the steps of melting and holding, casting, surface cutting and soaking, a scattering of a metal alloy component or the like is generated in surface layer of the aluminum support even if heating and cooling are repeated and surface-cutting for cutting the surface layer is performed, resulting in reduction of the yield of the aluminum support to be used as a planographic printing plate support.